There is, by now, a large literature on the chemistry and biology of love and sex. If you dive into that literature, you learn pretty quickly that our love lives are biased by all sorts of deep unconscious processes. When men become fathers, their testosterone levels drop, thus reducing their sex drive. There’s some evidence that it’s the smell of their own infants (but not other people’s infants) that sets this off.

Women, meanwhile, have different tastes at different times in their cycles. During ovulation, according to some research, they prefer ruggedly handsome and risky men, while at other times they are more drawn to pleasant-looking, nice men.

When men look at pictures of naked women, their startle response to loud noises diminishes. It seems that the dopamine surge mutes the prefrontal cortex, and they become less alert to danger and risk.

This literature sometimes reduces the profound and transformational power of love into a series of mating strategies. But it also, like so much of the literature across psychology and the cognitive sciences these days, reinforces a specific view of human nature. We have two systems inside, one on top of the other.

Deep in the core of our being there are the unconscious natural processes built in by evolution. These deep unconscious processes propel us to procreate or strut or think in certain ways, often impulsively. Then, at the top, we have our conscious, rational processes. This top layer does its best to exercise some restraint and executive function.

This evolutionary description has become the primary way we understand ourselves. Deep down we are mammals with unconscious instincts and drives. Up top there’s a relatively recent layer of rationality. Yet in conversation when we say someone is deep, that they have a deep mind or a deep heart, we don’t mean that they are animalistic or impulsive. We mean the opposite. When we say that someone is a deep person, we mean they have achieved a quiet, dependable mind by being rooted in something spiritual and permanent.

A person of deep character has certain qualities: In the realm of intellect, she has permanent convictions about fundamental things; in the realm of emotions, she has a web of unconditional loves; in the realm of action, she has permanent commitments to transcendent projects that cannot be completed in a single lifetime.

There’s great wisdom embedded in this conversational understanding of depth, and it should cause us to amend the System 1/System 2 image of human nature that we are getting from evolutionary biology. Specifically, it should cause us to make a sharp distinction between origins and depth.

We originate with certain biological predispositions. These can include erotic predispositions (we’re aroused by people who send off fertility or status cues), or they can be cognitive (like loss aversion).

But depth, the core of our being, is something we cultivate over time. We form relationships that either turn the core piece of ourselves into something more stable and disciplined or something more fragmented and disorderly. We begin with our natural biases but carve out depths according to the quality of the commitments we make. Our origins are natural; our depths are man-made — engraved by thought and action.

This amendment seems worth making because the strictly evolutionary view of human nature sells humanity short. It leaves the impression that we are just slightly higher animals — thousands of years of evolutionary processes capped by a thin layer of rationality. It lops off entire regions of human possibility.

In fact, while we are animals, we have much higher opportunities. While we start with and are influenced by evolutionary forces, people also have the chance to make themselves deep in a way not explicable in strictly evolutionary terms.

So much of what we call depth is built through freely chosen suffering. People make commitments — to a nation, faith, calling or loved ones — and endure the sacrifices those commitments demand. Often this depth is built by fighting against natural evolutionary predispositions.

So much of our own understanding of our depth occurs later in life, also amid suffering. The theologian Paul Tillich has a great essay in “Shaking the Foundations” in which he observes that during moments of suffering, people discover they are not what they appeared to be. The suffering scours away a floor inside themselves, exposing a deeper level, and then that floor gets scoured away and another deeper level is revealed. Finally, people get down to the core wounds and the core loves.

Babies are not deep. Old people can be, depending upon how they have chosen to lead their lives. Babies start out very natural. The people we admire are rooted in nature but have surpassed nature. Often they grew up in cultures that encouraged them to take a loftier view of their possibilities than we do today.